Excitation conditions in the Orion molecular cloud obtained from observations of ortho- And para-lines of H 2

Abstract

Aims. We seek to study excitation mechanisms in the inner region of the Orion Molecular Cloud by comparing observations of ortho-and para-lines of H 2 with theoretical models of slow shocks and photodissociation regions. Methods. K-band observations of H 2 obtained with the Canada-France-Hawaii 3.6 m telescope using the PUEO adaptive optics system are reported. Data were centered on the Becklin-Neugebauer object northwest of the Trapezium stars. Narrow-band filters were used to isolate emission from the v = 1-0 S(1) ortho- and v = 1-0 S(0) para-lines at a spatial resolution of 0.45 (∼200 AU). We are able to combine their intensity to obtain the column densities of rovibrationally excited ortho and para H 2 levels of the molecular gas at high spatial resolution. Results. The resulting line ratios show variations between 2 and the statistical equilibrium value of 6. We find 4 different classes of emission, characterised by the ratio of the v = 1-0 S(1) and S(0) line brightness and the absolute line brightness. Shock models are used to estimate the physical properties of pre-shock density and shock velocity for these 4 classes. We find that the pre-shock density is in the range of 10 5-10 7 cm -3 and shock velocities lie between 10 and 40 kms -1. Studies of individual objects, using additional constraints of shock velocity and width, allow quite precise physical conditions to be specified in three prominent bow shocks, one with a shock speed of 18 ± 2 km s -1 and pre-shock density 1 ±0.5 × 10 6 cm -3 (3σ) and two with shock speeds of ∼36± 2 km s -1 and pre-shock densities of 7.5 ± 2.5 × 10 4 cm -3. © ESO 2007.